First, only the defendant in this lawsuit can file a counterclaim, and the object of counterclaim is the plaintiff in this lawsuit.
Second, counterclaim must be based on the existence of this lawsuit. Without this lawsuit, there would be no counterclaim.
Third, the purpose of counterclaim is to attach and offset the plaintiff's claim, which can only be put forward before the judgment of first instance.
Fourth, the counterclaim should be filed with the people's court of this case.
Fifth, counterclaim has its independence. After the counterclaim is filed, if it is withdrawn, it will not affect the continued trial of the counterclaim.
Sixth, counterclaim and this lawsuit are based on the same fact or the same legal relationship.